The present invention relates to control systems for controlling power supplied to a dissipative/resistive load, and in particular, a power control system that protects an LED illumination array from reaching life-shortening or destructive temperature levels.
Sophisticated illumination systems and methods have been developed, for example, for use in the inspection of electronic components. One such illumination system, which is especially suitable for illuminating ball grid arrays (BGAs), which are commonly used in manufacturing electronic components, is disclosed, for example, in commonly-owned U.S. Pat. No. 5,943,125, which is fully incorporated herein by reference. U.S. Pat. No. 5,943,125 teaches the use of a ring-shaped light source, which includes a plurality of light emitting elements, such as light emitting diodes (LEDs). While this light source is designed especially for use in illuminating BGAs for inspection purposes, various configurations of LED arrays may be employed for a wide variety of illumination sources for a wide variety of inspection applications.
One drawback of using LED arrays as illumination sources, however, is that LEDs are dissipative (resistive) loads. As a dissipative/resistive load is powered, it will heat up. If the heat build up is allowed to progress uncontrolled, the temperature of the array may reach a destructive or life-shortening level.
Various systems and methods have been employed in the past to prevent dissipative/resistive loads from exceeding certain pre-defined life-shortening temperature levels. More sophisticated control systems have been employed as well to ensure that the peak and average temperatures of the LED array fall within safe limits. One such system controls the temperature of an LED array by enforcing a maximum pulse width of an LED power signal (during which the LED array is powered) and a minimum off time between pulses. This type of control system employs a simple digital circuit that generates a delay after each pulse.
A slightly more sophisticated prior art system computes an inter-pulse minimum delay based on the then-current pulse width. An even more sophisticated prior art system even takes the pulse repetition rate into account.
Since all of the prior art control systems are based on theoretical average thermal characteristics, they do not take into account the real-time, actual heat generation of an LED array. Therefore, a margin of safety must be factored into all prior art control systems. These built-in safety margins necessarily reduce the actual time of array illumination, which in turn limits the throughput of the inspection systems with which they are associated.
One solution to the problem with prior art control systems is to provide a power control circuit suitable for use in controlling dissipative/resistive loads (e.g., LED illumination arrays), which accurately models the heat being generated by the resistive load that it is controlling. In this manner, arbitrary, built-in safety margins can be eliminated, which provides an improvement in inspection system throughput. It also makes it possible to input a complex series of pulses of varying widths and intervals, such that power to the LED array could be arbitrarily switched without restriction, provided the modeled maximum temperature limit was not exceeded.
The control circuit discussed above, however, requires carefully calibrated and accurate low leakage analog components, especially when temperature calculations require a large ratio of charge (heating analog) to discharge (cooling analog) time constant. The analog control circuit for modeling temperature can thus be costly and requires careful layout and component selection.
Accordingly, there is a need for a power control system and method that models temperature with minimal or no analog components.
According to one aspect of the present invention, a power control system for controlling power supplied from a power source to a resistive load to prevent the resistive load from exceeding a predetermined high temperature limit. A regulator circuit is coupled between the power source and the resistive load for supplying controllable power levels to the resistive load. The power control system comprises a pulse train generating circuit for converting power impulses received from the regulator circuit into a heating pulse train representing power flowing to the resistive load. A load temperature calculation circuit is coupled to the pulse train generating circuit. The load temperature calculation circuit includes digital logic for producing a temperature out value substantially representing a present temperature of the resistive load.
A temperature comparison circuit is coupled to the load temperature calculation circuit and the regulator circuit. The temperature comparison circuit selectively compares the temperature out value to at least one of a high temperature limit value and a base temperature value. The temperature comparison circuit causes the power source to be disconnected from the resistive load when the temperature out value reaches the high temperature limit value. The temperature comparison circuit causes the power source to be reconnected to the resistive load when the temperature out value reaches the base temperature value.
According to one embodiment of the power control system, a pulse rate generator circuit including one or more oscillators generates heating and cooling pulse rates. An AND gate receives the heating pulse rate from the pulse rate generator circuit and receives a power control pulse from the regulator circuit. The heating pulse rate and the power control pulse cause the AND gate to output a heating pulse train such that the number of pulses out of the AND gate is proportional to the total energy delivered to the resistive load.
An up/down counter is coupled to the pulse rate generator circuit and receives the heating pulse train, which is applied to an up input of the up/down counter. The up/down counter outputs a temperature out value substantially representing a present temperature of the resistive load. A rate multiplier is coupled to the up/down counter and to the pulse rate generator circuit for generating a cooling pulse train, which is applied to a down input of the up/down counter. A temperature comparison circuit receives the temperature out value and provides a power control signal to the regulator circuit to disconnect or re-connect the power source.
According to one method of controlling power supplied from the power source to the resistive load, a heating pulse train representing power flowing to the resistive load is generated. Load temperature is modeled using digital logic and the heating pulse train to generate a temperature out value substantially representing a present temperature of the resistive load. The temperature out value is compared to a high temperature limit value, and the power source is disconnected from the resistive load if the temperature out value exceeds the high temperature limit value. The temperature out value is compared to a base temperature value, and the power source is re-connected to the resistive load if the temperature out value reaches the base temperature value.